A hallmark of microbes is the ability to alter their physiology to tolerate or exploit local conditions. Adaptation is crucial for pathogens, since these microorganisms confront distinct environments as they cycle between periods of colonization and transmission. As an experimental model to elucidate the impact of microbial differentiation on pathogenesis, we analyze a gram-negative intracellular pathogen of phagocytes. Here we test the hypothesis that, to thrive in the human lung and its aquatic reservoir, L. pneumophila expresses in a reciprocal pattern at least two distinct sets of biochemical pathways: one to promote replication in phagocyte vacuoles and the other to stimulate transmission between host cells. The hypothesis that its LetA/S two- component regulatory system is designed to confer versatility to the pathogen will also be tested. By exploiting knowledge of the regulatory circuit that governs L. pneumophila differentiation, mutants that lack the regulators FliA, LetA, LetS, or CsrA, or the threonine tranporter PhtA, the genome sequences of the Philadelphia, Paris and Lens strains, bioinformatic and microarray technology, and molecular and genetic assays of function, mechanisms that equip pathogens to cycle between intracellular replication and transmission can be elucidated. The molecular features critical to each stage of the life cycle can inform design of agents to eradicate this opportunistic pathogen from contaminated water supplies and from the infected lung. Together, these studies will provide a framework to investigate the biochemical pathways that equip intracellular microbes to emerge from the environment to cause opportunistic infections. Thus, concepts and pathways identified here can also guide studies and management of less tractable intracellular pathogens such as the Mycobacteria, Chlamydia, Francisella, and Coxiella species.